Conventionally, a method that utilizes principles of white light interference has been proposed for measuring surface height of an object precisely and in a non-contact manner. In such a method, a light beam emitted from a white light source is split into a measurement light flux that is directed to the object to be measured and a reference light flux that is directed to a reference mirror, and after being reflected from the surface of the object and from the reference mirror, respectively, the two light fluxes are combined again into one light flux to be detected. The amplitude of the white light interference fringes becomes maximum when the optical path length of the measurement light flux is equal to the optical path length of the reference light flux. Thus, by moving the reference mirror and measuring the position of the reference mirror when the amplitude of the interference fringe is maximum, and thereby determining the optical path length of the reference light flux, it is possible to determine the surface height of the object to be measured.
White light interference can be observed only if the two light fluxes described above have substantially equal optical path length. Therefore, as the range of dimension to be measured is large, the range of movement of the reference mirror becomes large. Further, as the range of movement is large, the time required for movement of the reference mirror over the entire range becomes long, so that the measurement time required for each point to be measured on the surface of the object becomes long, too. Therefore, in the measuring method that utilizes white light interference, if the surface area of the object to be measured is large and a large number of points have to be measured, a very long time may be required for measurement of the entire object. Thus, a mechanism is disclosed, for example, in Tatsuo Shiina, et al. “Long optical path scanning mechanism for optical coherence tomography”, Applied Optics, Vol. 42, No. 19, pp. 3795-3799, 2003, in which a plurality of corner cubes are mounted on a rotary table as reference mirrors, and by rotating the rotary table at high speed and thereby moving the corner cubes at high speed, the optical path length of the reference light can be varied at high speed.
In the mechanism as disclosed in the above publication, however, if the position of the rotation axis of the rotary table is shifted, the optical path length of the reference light flux changes accordingly, and therefore, it is difficult to measure dimensions of an object in high precision.